Hello A.,
Style for gantt diagram is locally, then you can add a lot of style during the task definition as:
@startgantt
<style>
task {
FontName Helvetica
FontColor black
}
</style>
Project starts 2020-11-13
[One] lasts 7 days
then [Eight] lasts 1 days
<style>
task {
FontColor red
}
</style>
then [Nine] lasts 4 days
<style>
task {
FontColor black
}
</style>
then [Ten eleven] lasts 1 days
then [Twelve] lasts 4 days
@endgantt
If that can help,
Regards,
Th.